1. Field of the Invention
The present invention relates to an optical communications system, and devices for use in such a system, and methods of protecting an optical route. In particular, but not exclusively, the invention relates to an optical fuse for protecting network components in the route.
2. Description of Related Art
Optical communication routes are known including optical waveguides, such as optical fibres and optical components, such as lasers and optical amplifiers. Such routes have a huge information-carrying capacity.
It is known that an optical route can suffer damage as a result of power surges. For example, routes which include rare earth doped optical amplifiers can suffer damage. In particular, when a rare earth doped optical amplifier goes from a state in which no signal is being input to a state in which an input signal is received, the device may output a surge of power. Under certain circumstances such power surges may cause damage to the route including the amplifier and devices downstream of the source of the power surge.
In such circumstances, it may be necessary to protect optical routes from power surges.
EP 0 943 954 proposes a solution to this particular problem. An optical fuse is provided, which is disposed in an optical route, and operates along principles similar to an electrical fuse. The fuse is arranged to “blow” when optical radiation travelling in a downstream direction away from an optical source, incident on the fuse is in excess of a preselected threshold level chosen by the route operator. The threshold is set so that under normal operating conditions the fuse will not “blow”, but will “blow” in the event of a power surge.
Once the fuse has “blown”, the power surge radiation can no longer propagate beyond the “blown” fuse. The fuse is disposed between two optical fibres, and includes a first layer which heats up on exposure to optical radiation and a second layer which, depending on which material is chosen, loses its transparency or reflectivity, respectively, when heated. The layers are dimensioned and arranged so that when optical radiation above the preselected threshold is incident on the first layer, sufficient heat is generated to cause the second layer to lose its transparency or reflectivity, depending on the material chosen. Thus, the optical signal can no longer propagate from the first to the second optical fibre. The fuse cannot be reset. Once the fuse has “blown” it must be replaced before the route can work normally. Provided the power level remains below the preselected threshold level the fuse remains transparent or reflective, thus allowing the further propagation of an optical signal.
One drawback of the fuse proposed in EP 0 943 954 is that the devices are complex to fabricate. Further, the fuse described in EP 0 943 954 has been designed for the particular application described above of arresting the propagation of a power surge. However, it has been found that in addition to power surges, an optical fibre carrying an optical signal may be subject to other optical phenomena that may damage the fibre.
As a result of an optical communications route's huge information-carrying capacity, there is a growing demand on optical communications routes to carry an increasing amount of information. There is therefore much interest in developing methods for increasing the transmission capacity for optical routes. One method is wavelength-division multiplexing (WDM), in which several data channels, at different wavelengths, are carried simultaneously on the same fibre. Thus, the information carried by the waveguide, and also the amount of power transmitted by the waveguide is increased in accordance with the number of channels carried by each fibre.
Although, it is known, that optical waveguides are able to transmit high power signals without suffering any damage, it has been found that an optical waveguide may undergo catastrophic self propagating damage if, whilst transmitting a high power signal, it is subject to an external stimulus. Such stimuli are discussed in Electronics Letters, 2 Mar. 2000, vol. 36, no. 5, pages 414–416, and may, for example, be breaking or cutting of the waveguide, but may also be some externally applied mechanical shock which does not itself interrupt the fibre path, such as for example, bending the optical fibre. This effect is called “self propelled self focussing damage”, and is the subject of EP 0 309 234. The mechanism may be started by accidental damage to a fibre cable carrying in excess of approximately the order of 1 Watt of optical power, and has been seen when a broken fibre end comes into contact with an absorbing surface. The damage can be observed as a blue-white plasma-like localised emission which travels at a velocity of tens of centimeters per second or higher in an upstream direction back along the fibre towards the source of the optical power. The power required to sustain the propagation of the damage is “fuelled” by the optical signal. The damage will thus propagate along the fibre in a direction towards the source of the optical signal, feeding on the optical signal. The damage will continue to propagate as long as the fibre carries a signal having a power above approximately the order of 1 Watt (the precise power level required to cause and sustain such damage depends upon the properties of the fibre). In WDM systems, the combined optical power of the data channels may be above 1 Watt. As mentioned above, damage may occur if a fibre transmitting more than approximately 1 Watt is subject to an external stimulus, and hence, there exists a real risk of this catastrophic damage in such WDM systems.
One disadvantage of the optical fuse described in EP 0 943 954 is that it would not be suitable in the situation described above. The fuses in EP 0 943 954 are suitable for arresting an optical power surge travelling in a downstream direction from damaging any optical components disposed downstream of the origin of the power surge. They are designed to blow when an optical amplifier experiences a surge in optical power. They are designed not to blow under operating conditions at which the optical route was designed to operate. Self propelled self focussing damage however, occurs at normal high power transmission conditions and results in catastrophic damage to the optical route which propagates in a direction upstream of the origin of the self propelled self focussing damage.
A fuse suitable for arresting damage must not blow at the operating optical power of the system, but must prevent propagation of damage beyond the site where the device is located should damage occur.
Although, it is likely that the fuse of EP 0 943 954 would blow if it were subject to catastrophic damage, it is not suitable because it would not arrest the damage because the source of optical power fuelling the damage would not be interrupted. Indeed, the fuse of EP 0 943 954 might even initiate self propelled self focussing damage due the heat generated when it blows as the result of a downstream propagating power surge. Whilst the damage propagated through the fuse, it would continue to be fuelled by the optical signal being transmitted in a downstream direction in the opposite direction from the propagating damage. The damage would not be arrested until the optical signal was turned off. The only solution would be to ensure that the systems in which the fuses of EP 0 943 954 operated at optical powers below the threshold power above which catastrophic damage occurs. As a result of the increased demand for higher capacity on optical fibres, operating optical communication routes below this threshold may not always be desirable.
U.S. Pat. No. 4,973,125 discloses a self limiter for fibre optics. The limiter includes a semiconductor optoelectronic device. Between the device and the system fibre an air gap is provided. The size of the gap determines the self limiting effect provided. U.S. Pat. No. 6,014,396 discloses a flared semiconductor optoelectronic device. The devices disclosed in U.S. Pat. Nos. 4,973,125 and 6,014,396 suffer from the disadvantage that the semiconductor devices must be packaged in such a way to enable their integration into the optical fibre system. Further, U.S. Pat. No. 6,014,396 suffers from the additional disadvantage that the device must be constructed and packaged in such a way that the dimensions of the air gap are accurately established and maintained.